Better editor background

Description

When the editor body has max-width (most newer themes set that) the body in the TinyMCE iframe is aligned left or right (RTL), and the content suddenly stops in the middle (if the browser window is wide). This doesn't look good and is a bit weird and unexpected.

Yeah, tried several similar styles. One problem with them is that when inserting a floated image the body doesn't stretch and looks "broken". Another problem is browser differences: without using background-image the css needs 4 variations to cover all supported browsers (yeah, IE7 too).

A nice way of doing this is to create a "flat" gradient for background-image with css3 instead of using a real image. Not working in IE < 10 though.